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Employee Share-Based Incentive Compensation Plans (Restricted Shares) (Details) - W T M Incentive Plan - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
White Mountains | Restricted Share Cliff Vesting in January 2021 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) | 13,450 |  |  |
White Mountains | Restricted Stock |  |  |  |
Target Restricted Share Activity |  |  |  |
Beginning of period (in shares) | 41,510 | 53,815 | 53,755 | 70,620
Issued (in shares) | 0 | 0 | 14,105 | 17,785
Vested (in shares) | 0 | (260) | (25,381) | (28,846)
Assumed forfeitures and cancellations | 0 | 0 | (969) | (6,004)
Ending of period (in shares) | 41,510 | 53,555 | 41,510 | 53,555
Unamortized grant date fair value |  |  |  |
Unamortized grant date fair value, beginning balance | $ 19.0 | $ 23.5 | $ 14.3 | $ 19.7
Issued | 0.0 | 0.0 | 11.4 | 16.7
Assumed forfeitures and cancellations | 0.0 | 0.0 | (0.2) | (3.5)
Expense recognized | (3.2) | (3.8) | (9.7) | (13.2)
Unamortized grant date fair value, ending balance | $ 15.8 | $ 19.7 | $ 15.8 | $ 19.7
White Mountains | Restricted Share Cliff Vesting in January 2019 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) | 365 | 250 |  |
White Mountains | Restricted Share Cliff Vesting in January 2018 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) |  | 250 |  |
White Mountains | Restricted Share Cliff Vesting in January 2020 |  |  |  |
Target Restricted Share Activity |  |  |  |
Issued (in shares) | 290 | 550 |  | 16,735
Sirius Group | Restricted Stock |  |  |  |
Target Restricted Share Activity |  |  |  |
Ending of period (in shares) |  | 2,195 |  | 2,195

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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